By Terry Eliasen, Meteorologist, WBZ-TV Exec. Weather Producer

BOSTON (CBS) – It’s one of our biggest fears as meteorologists – a tornado touchdown in the middle of the night with relatively little warning.

That is exactly what happened just before 3 a.m. Thursday morning in parts of Worcester County.

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It wasn’t your classic squall line of thunderstorms – far from it in fact. The storm, which produced two EF-1 tornadoes, initially looked very innocuous; just a heavy downpour in the middle of a large area of rain.  Just before 2 a.m., the downpour stated to blossom a bit as it passed along the border of northeast Connecticut and northwest Rhode Island.

As it crossed the border into Douglas, we got our first warning sign that perhaps this was going to be more than just another heavy downpour. The radar image at 2:20 a.m. showed a small appendage on the lower right side of the cell, and our velocity scans showed some rotation developing within that area. Just before that, the Storm Prediction Center issued this statement and image:

(Storm Prediction Center Alert)

There is a lot of technical mumbo-jumbo in there but essentially what they were saying was “Hey, this isn’t your typical setup for a tornado, there is no lightning and the storm is embedded within a large area of rain, BUT there is just enough spin (Helicity) with this thing that we cannot rule out a brief tornado.”

(Image Credit: Storm Prediction Center)

And sure enough, minutes later, many of the folks in Douglas and Upton were woken up to the sound of torrential rain and howling winds. Trees were snapped and some structures were damaged.

(WBZ-TV Graphic)

(WBZ-TV Graphic)

Check out this radar image from 2:44am as the cell was entering Upton. That appendage, still there and in fact a bit more pronounced than in Douglas. More reports of damage and trees being snapped flooded in.

(WBZ-TV Graphic)

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Than a few minutes later, in Marlboro, this warning happened:

308 AM: Marlboro, MA: several large limbs down and lawn furnishings scattered around rt20 and concord rd Marlboro and on Marlboro country club course on Concord rd and a 45 mph sustained tstorm wind

The same cell from Douglas and Upton, but no longer any obvious circulation. The damage in Marlboro was likely caused by straight-line wind damage and not a tornado.

(WBZ-TV Graphic)

The National Weather Service sent out a survey team to the area to investigate exactly what happened last night. The team confirmed two EF-1 tornadoes, both with max winds around 100 mph. The first was tracked from Douglas, through Uxbridge and Northbridge with a length of 4.4 miles and a width of 200 yards. The second was in Upton with a length of one mile and width of 100 yards.

Path of tornadoes in Worcester County (WBZ graphic)

Tornado damage is fairly easy to differentiate from straight-line wind damage. The debris from trees and structures will be laid out in a circular fashion, indicating a twisting of the winds as opposed to straight-line wind damage, which lays all the damage out in the same direction.

Tree damage in Upton following an overnight storm. (WBZ-TV)

These weak, “spin-up” tornadoes within a large area of rain are very hard to detect and come and go so quickly, they often go unwarned. A very similar occurrence happened nearly four years ago to this date in Revere. Another rain-wrapped, spin-up tornado, this one just after 9 a.m., was also unwarned and caused significant damage along a 2-mile path.

We are fortunate here in the Northeast in comparison to many other parts of the country. We very rarely get any major tornadoes; most of ours are quite weak and short-lived. However, they also can be much harder to predict and detect given our hilly topography and the rain-wrapped nature of many of our storms. A night like last night serves as a reminder that we can never let our guard down. As far as technology has come, Mother Nature remains somewhat unpredictable.

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Follow Terry on Twitter @TerryWBZ